1. Field of the Invention
The present invention relates to a quantum well structure, more particularly to a quantum well structure capable of efficiently confining electrons and holes, and to a semiconductor laser utilizing such a quantum well structure.
2. Description of the Related Art
In a semiconductor light emitting device such as a semiconductor laser and a light emitting diode, electrons and holes injected into an active layer of the device combine with each other and emit light. Accordingly, it is important to efficiently confine the electrons and holes in the active layer in order to improve characteristics of the light emitting device. A conventional semiconductor laser has a double hetero structure, in which an active layer is sandwiched between a p-type cladding layer and an n-type cladding layer. Compound semiconductors for forming the double hetero structure are selected so that the forbidden gap of the active layer is smaller than the forbidden gap of the p-type cladding layer and the n-type cladding layer. This energy difference (.DELTA.Eg) between the forbidden gaps generates energy barriers as the band offset (.DELTA.Ev) of the valence band and the band offset (.DELTA.Ec) of the conduction band. These energy barriers, when sufficiently high, can achieve efficient confinement of the injected electrons and holes in the active layer.
Recently, owing to developments in crystal growth techniques, it has become possible to grow an ultra-thin film having a thickness of several nanometers (nm). Thus, a quantum well semiconductor laser can be manufactured using such an ultra-thin film as an active layer of the laser. In a quantum well layer (i.e., an active layer made of an ultra-thin film by which quantum effects of electrons and holes are obtained) of the quantum well semiconductor laser, electrons and holes each have a discrete energy level. As a result, the quantum well semiconductor laser has advantages such as a decrease in the threshold current density due to an increase in the state densities, emission of laser light having a shorter wavelength, and the like.
An example of conventional semiconductor lasers includes a red semiconductor laser which is made of AlGaInP/GaAs type compound semiconductors and has an emission wavelength in the range from 630 to 670 nm. An attempt has been made to further shorten the emission wavelength of the red semiconductor laser by applying tensile strain to a quantum well layer. This tensile strain is generated by forming the quantum well layer from a compound semiconductor having a smaller lattice constant than those of compound semiconductors used for the cladding layers and the substrate. However, it is believed that in such a red semiconductor laser, the band offset (.DELTA.Ec) of the conduction band is not sufficiently large. Thus, when the amount of injected carriers (electrons and holes) is increased in an operation at a higher temperature, the electrons tend to overflow from the quantum well layer to the p-type cladding layer, thereby deteriorating the characteristics of the semiconductor laser.
Another type of quantum well semiconductor laser includes that which is made of group II-VI semiconductors and recently reported to successfully emit a blue light. Such a blue semiconductor laser also has a similar problem to that mentioned above. For example, in the blue semiconductor laser in which a quantum well layer, cladding layers and a substrate are respectively made of ZnMgSSe, ZnSSe and GaAs, neither the band offset (.DELTA.Ev) of the valence band nor the band offset (.DELTA.Ec) of the conduction band is sufficiently large. Also, in the case of a quantum well semiconductor laser in which group VI elements included in a quantum well layer are the same as those included in the cladding layers, such as a semiconductor laser having a quantum well layer, cladding layers and a substrate respectively made of ZnCdMgS, ZnCdMgS and GaAs, the band offset of the valence band (.DELTA.Ev) is very small. Because of this small .DELTA.Ev, the injected holes tend to overflow from the quantum well layer to the n-type cladding layer, resulting in difficulty of the semiconductor laser in oscillating at a higher temperature.
In order to overcome the above-mentioned problems, a semiconductor laser having a structure as shown in FIG. 8 has been proposed (Japanese Laid-open Publication No. 4-180684). In this semiconductor laser, an n-type carrier confining layer 803 is disposed between an active layer 804 and an n-type cladding layer 802, and a p-type carrier confining layer 805 is disposed between an active layer 804 and a p-type cladding layer 806. The carrier confining layers 803 and 805 are each made of (Al.sub.0.6 Ga.sub.0.4).sub.0.6 In.sub.0.4 P, and each has tensile strain due to a lattice mismatching. Because of these mismatchings, an energy gap of each of the carrier confining layers 803 and 805 is enlarged without being accompanied by an increase in the specific resistance thereof. However, the thickness of each of the carrier confining layers 803 and 805 is limited to such a thickness as to avoid the elastic breakdown (i.e., a thickness not greater than the critical thickness), and thus it is extremely difficult to prevent the electrons from over-flowing to the p-type cladding layer due to the tunnel effect. Therefore, even the semiconductor laser of FIG. 8 is believed to be unsatisfactory regarding the efficiency of confining the carriers.